Low pressure fluorescent and discharge lamps



Aug- 2, 1955 G. MEISTER ET AL LOW PRESSURE FLUORESCENT AND DISCHARGE LAMPS Filed June 27, 1952 INVENTORS 6507965 ME75' United States Patent LOW PRESSURE FLURESCENT AND DISCHARGE LAMPS.

George Meister, Newark, and Thomas H. Heine, Cedar Grove, N. J., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Penn- Sylvania Application June 27, 1952, Serial No. 296,042 2 Claims. (Cl. 313-409) This invention relates to low pressure discharge lamps and, more particularly,`to such employing only mixtures of rare gases and mercury as the filling material.

The principal object of ourinvention generally considered, is to produce low pressure iluorescent and discharge lamps having a iilling of a mixture of mercury and rare gases of such a composition that fluorescence is eiciently excited` in` the phosphor.

Another object of our invention is to produce a low pressure iiuorescent or discharge lamp comprising a mixture of rare gases in which xenon ismixed with argon and mercury in the proportion to get good etliciency at practical pressures.

A further object of our invention is to produce a fluorescent or discharge lamp which has characteristics such as above outlined, whereby it is better than a lamp having only one rare gas admixed with mercury vapor because it has a high ultra-violet and corresponding luminous efficiency.

Other objects and advantages of the invention will become apparent` asthe description proceeds.

Referring to the drawing:

Figure 1 is an elevational view, with parts in longitudinal section of a lamp embodying our invention.

Figure 2 is a graph showing the variation in operating voltage as the composition of the contained gas is changed, each curve having applied thereto` a numeral indicating pressure in` millimeters of the enclosed gas.

Figure 3 is a graph showing the variation in arc or visible light output eiiciency as the composition of the contained gas is changed, each curve havingapplied thereto a numeral indicating pressure in millimeters of the enclosed gas.

Figure 4 is a graph showing the variations in ultraviolet output eiiiciencyl as the composition of the contained gas is changed, each curve having applied thereto a numeral indicating pressure in millimeters of the enclosed gas.

Figure 5 is a graph showing the variations in fluorescent light output eticiency` as` the composition of the contained gas is changed,` each curve` having applied thereto a numeral indicating pressure in millimeters of the enclosed gas.

As is well known, mercury vapor admixed with an inert or rare gas, such asargon atlowt pressure, iscom mercially employed for the generation of` ultrafviolet radiations which may excite phosphors` toi give oli visible radiations in fluorescent discharge lamps.

The electrical characteristics of such low. pressure mercury discharges are known. It-has beenshown` that, at a given lamp temperature, pressure and current,` as the atomic weight of the inert gas increases, the voltage gradient in the mercury discharge and the voltage drop at the electrodes` decrease.

v The early data` was mainly taken on the individual inert gases mixed with mercury. We have investigated the use of.` mixtures of;- inert gases with mercury for discharge lamps, as a` continuation` of the `subject matter described and claimed in our applications, Serial No. 102,016, led June 29, 1949, and Serial No. 243,612, tiled August 25, 1951, and owned by the assignee of the present application.

In order to obtain as much information as possible on one experimental lamp, a special tube was designed in which the central portion consisted of a 5 inch section of glass identified by the Corning trademark Vycor" (as further identied in the Rentschler Patent No. 2,469,410, dated May 10, 1949) and of 1.5 inches inside diameter which was sealed to a 1.5 inch diameter glass tube, identified by the Corning trademark Pyrex, at each end. The Pyrex sections could be coated with phosphors. Independently heated oxide coated cathodes were used and spaced about 24 inches apart. In addition, a water-jacket three inches in diameter was designed to t around the lamp in which the central portion again was a 5 inch Vycor section sealed with white wax to a Pyrex section at each end. With this construction it was` possible to measure the light output of the phosphors, the ultra-violet output of the arc discharge, and the light output of the arc, at different gas pressures ano' controlled temperatures with corresponding voltages and power input.

The assembled tube with coated phosphor sections Was sealed onto an exhaust system which had a cooling trap, surrounded by Dry Ice throughout the experiment. A liter reservoir for allowing proper diffusion of gas mixtures was provided into which could be introduced the spectroscopically pure inert or noble gases which were employed. The system also was equipped with a Mc- Leod gauge to read pressures.

The lamp was exhausted and baked at 475 C. for about one hour. The cathodes were processed until no more gas was liberated. The water-jacket was put into place so that the Vycor sections of the jacket and lamp Vycor sections coincided and was rnily fastened to keep it in a lixed position. The annular space between the lamp and jacket could be flushed with Water at any desired temperature, which was read on a thermometer placed in the water surrounding the lamp. A voltage stabilizer was used to control the voltage input on the filament transformers, the discharge transformer, and the ultraviolet meter.

The ultra-violet meter containing a` tantalum cell to readl` the 2537 radiation was set in position opposite the Vycor section. The photovaltaic cells were placed in` position opposite the Vycor section and the phos phor sections. They were checked before and after a set of readings with a standard incandescent lamp, also using a voltage stabilizer in the circuit. All stands were securely fastened so as to keep all positions ixed during a run.

The lamp was seasoned` for several days before any readings were taken. This seasoning was done in mercury vapor alone, continuously exhausting during the entire period. During operation of the lamp, water was flushed up and down the tube until the water temperature was 45 C. All the data were subsequently taken at this temperature. The temperature was checked before and after each current reading. As each current setting was made, the ultra-violet output of the arc, the voltage, the visible light output of the arc, and the light output of the phosphor were read in that order. These zero gas pressure data were again obtained as a reference check after each change of inert gas or inert gas mixture. After all the preliminary data were obtained, then this lamp was operated atdilferent inert gas pressures and mixtures of inert gases. The inert gases investigated were xenon, argon and mixtures of xenon and argon.` These gas mixtures were varied so as to obtain sucient data to show any diiierence in characteristics and each one ing more than 25% xenon.

Xenon Argon Percent All data illustrated in the accompanying composition curves are for a constant lamp current of 500 milliamperes and for a constant temperature of 45 C. for diierent pressure parameters ranging from l to mm. As in the earlier applications, the output of krypton at 2 mm. gas pressure is used as the standard to set the 100% reference point.

VOLTAGE CHARACTERISTICS Curves plotted to compare the voltage with the corresponding composition at constant current and temperature for xenon-argon mixtures show that the lowest voltage is obtained with 2 mm. gas pressure throughout the entire range of composition from 100% xenon to 100% argon. These curves, therefore, generally agree with previous data obtained with the krypton-argon and krypton-neon mixtures. However, in the neon-rich krypton-neon mixtures, the lowest voltage was obtained with 1 mm. gas pressure.

From Figure 2 it can be seen that minimum voltage occurs at 2 mm. throughout the entire range. For pressures at and below 3 mm., the curves are slightly concave upward, but at higher pressures a point of inection develops near the 40% xenon 60% argon composition. At 5 mm. the curve shows that the addition of xenon to argon has relatively little effect until more than 20% xenon is added. In contrast, the addition of a small percentage of argon to xenon has a pronounced effect. At lower pressures, the addition of xenon to argon has a much greater effect than the addition of argon to xenon.

The voltage of the gas pressure for to approximately 95% 2 mm., while for 2 mm., gas pressure it is approximately 90.5%. Also, the voltages for 1 and 3 mm. gas pressures are approximately the same throughout the xenon-argon series at each pressure and vary only from about 95% at pure xenon to about 132% at pure argon.

A statement was made that the voltage in these low pressure mercury discharges containing inert gases varies directly with the ionization potential of the gas. This relation is shown by plotting the voltage against. the atomic number of the gas which is correlated to the ionization potential. This correlation is further shown when the relative percentage voltage values and ionization potential values are tabulated with krypton as the reference point (Table 1), at 2 mm.

for pure xenon is practically independent 1, 3, 4 and 5 mm. and amounts Table 1 Atomic Percent Ionization Gas Number VOlage Potential Percent ARC CHARACTERISTICS Figure 3 shows the arc etliciency for xenon-argon xenon increases. At the lowbegins for mixtures contain- Here again a table very mm., as the percentage of est pressure, the decrease of the voltage of pure krypton at It is interesting to note that the arc visible output efficiencies for pure xenon show approximately the same large decrease as later observed in the ultraviolet and uorescent-composition curves, shown respectively in Figures 4 and 5. On the other hand, pure argon at all pressures from 1 to 5 mm. maintains a high efficiency showing a drop of only 14% over that pressure range. These curves, for pressures down only to 1 mm. compare with the curves previously published on the kryptonargon series which also showed the highest eiciency at 1 mm. pressure. However, at pressures below 1 mm., the efficiency of a 75% xenon 25% argon mixture reaches a maximum which is higher than the corresponding values for other pressures. The following table presents the maximum Vvisible output eciencies and the pressures at which they occur, for representative mixtures with krypton at 2 mm. taken as the 100% reference point It is noted that the maximum occurs at 0.2 mm. gas pressure for pure xenon and then shifts gradually up to 1.0 mm. gas pressure for pure argon. This is similar to the characteristics previously published on krypton-neon mixtures which showed that the gas pressure for maximum efficiency gradually rose from 1 mm. for pure krypton up to approximately 2 mm. for pure neon. In the case of krypton-argon there was no shift in gas pressure for maximum efficiency. In connection with these maximum arc eiciencies, it should be noted that they do not occur at the pressure for minimum voltage which is the case with ultra-violet and iluorescent output efficiencies.

ULTRA-VIOLET CHARACTERISTICS Figure 4 shows that at 2 mm. the U. V. efciency rises from about 94% for pure argon to a maximum of 97% between 50% and 60% xenon, and then drops back to about 94% for pure xenon. The 1 mm. pressure curve rises from a lower efciency for pure argon to a maximum of nearly 98% at 80% xenon. At 3 mm. and higher pressures, the addition of substantial proportions of xenon causes a decrease in efficiency.

FLOURESCENT CHARACTERISTICS The uorescent output efticiency vs. composition curves at constant current and temperature for xenonargon mixtures (Fig. 5) have the same general shape and relative output as the ultraviolet eiiiciency vs. composition curves at the corresponding gas pressures.

The highest uorescent output efficiency is obtained at approximately 1 mm. gas pressure for pure xenon and for mixtures down to about 60% xenon-40% argon. Below this composition the highest fluorescent efficiency occurs at 2 mm. gas pressure, and the efficiency then progressively decreases as the pressure increases.

The fluorescent output efficiency for pure argon is better than that for pure xenon at all pressures of 3 mm. and above. As the pressure increases from 3 to 5 mm., the

While here 1 mm. isv

5 fluorescent output efiiciency for the mixtures far exceeds that of pure xenon, so. that at 5 mm. the iluorescent eflciency is only about 66% that. ofxenon at l mm. The relative decrease in fluorescent output efficiency is ap- 6 i. e., it; has, enough wattage input tomaintain the vapor pressure of mercury for best ultraviolet and fluorescent; efficiency. All these gas mixturesVV at; 1. gas. pressure will equal or exceed the maximum ultraviolet and fluor proximately the same as with the ultravviolet efficiency. 5 reseent etliciency obtained at; 2 mm. gas pressure with a. So far, the matter of voltage, are, ultra violet or fluo- 50% Xe-5Q%= A mixtur Over the range where these. rescent output only, have been discussed. We now come gas mixtures; have. this high ultraviolet and fluorescentV to the bearing that voltage has o n fluorescent light.` output. efficiency, the voltage increases gradually and generally. A consideration of the voltage curves will show that they linearly going frompure Xe to 50% Xie-50% A. Thus; increase toward the pure argon end of the range, thereby 10. while the voltage increases gradually, the ultraviolet and effecting a corresponding increase in total output as the fluorescent eflciencies for these gas mixtures are anomapercentage of argon is increased, even though the ellous both at 1 mm. and 2` mm. gas pressure. ciency is not at a maximum, especially at the lower The arc eiciency, which is the eeieney Of the visible pressures. This effect is illustrated in the following: light output of the discharge, is highest at 1Y mm. gas T able3 Relative total light output of Xe-A mixtures, from Figs. 2 and 5. Pressure 1 mm. 2 tum. 3 mm. i 4 mm. Xe A Output V VA I Total Output V I VA Total Output V I VA Total Output V VA Total Percent Percent From the foregoing table, it will be seen that since the luminous fluorescent efhciency, with 2 mm. total gas pressure for a 20% xenon-80% argon mixture, is practically the same as pure xenon, the less expensive argon can be substituted up to `80%, and thereby obtain a total light output which is about 31% higher than that from pure xenon, and about 12% higher than the approximately 50% xenon-50% argon mixture giving maximum 1uminous elllciency.

Similarly, when the total gas pressure is l mm., a 40% Xenon-60% argon mixture would have a total light output which is about 17.5% higher than when using pure xenon, and about1l% higher than the light output of an 80% xenon-20% argon mixture where the highest elliciency is approached. A xenon-90% argon mixture at 2 mm. pressure will give an additional 3.7% total light output over that of the xenon-80% argon mixture, and at 1 mm. gas pressure the total light output would be about 18% higher than that for the 80% xenon-20% argon mixture. These argon-rich mixtures, therefore, would not only light output, but such would be obtained at only a slight sacrifice in the luminous efliciency obtainable from these xenon-argon mixtures.

` SUMMARY In accordance with the second of the previously-mentioned applications, it was found that xenon-krypton (Xe-Kr) gas mixtures containing mercury when operating at 45 C. (ca. 10p Hg vapor) had a maximum ultraviolet and corresponding fluorescent efciency at 1 mm. gas pressure for a 75% Xe-25% Kr mixture. On further investigating Xe with other inert gases such as Argon (A) in the presence of mercury vapor, it was found that a maximum ultraviolet and fluorescent efhciency is obtained at 2 mm. gas pressure for about a 50% Xe-50% A mixture. It is further to be noted for commercial reasons that if l mm. gas pressure is suicient to obtain life maintenance in a discharge lamp containing mercury, then Xe-A gas mixture may be used up to 60% Xe-40% A and still obtain about the same high ultraviolet and fluorescent efficiency, while the corresponding voltage changes only about 10% going from pure Xe to 60% Xe40% A.

This increase in voltage at the latter concentration will thus bring it up to slightly above (ca. 6%) the operating voltage of a pure Kr discharge lamp at 2 mm. gas prcssure which is, in fact, a commercially available ilamp,

be less expensive and give more total i pressure over the entire Xe-A gas compositions. There is a slight indication of a peak at about 25% Xe75% A which is different in composition from where the Xe-A gas mixtures have the maximum ultraviolet and fluorescent eflciency, namely Xe50% A. Thus it was shown that at 2 mm. gas pressure the ultraviolet and fluorescent efficiency for a 50% Xe-50% A is higher than pure A or pure Xe, and that at 1 mm. gas pressure these eficiencies are higher than for pure A and are equivalent to that of pure Xe even if the gas contains 40% A (60% Xe-40% A). The voltage at both cited pressures is normal and regular for the compositions considered. The visible light-output of the arc at l mm. gas pressure is highest for all compositions of Xe-A. Since Xe-A mixtures can be used instead of pure Xe, these mixtures also would reduce the cost considerably of, for example, a fluorescent lamp where the expensive Xe could be partially replaced with A.

Fluorescent lamps, one of which is illustrated in Figure 1, exhibit these experimental data. The lamp of said figure comprises an elongated translucent vitreous envelope 11, with heated lilamentary electrodes 12 and 13, one in each end portion, and containing the selected noble gas mixture and some mercury, indicated by the globule 14. If a lluorescent lamp, the selected phosphor 15 is applied to the inner surface of the envelope.

Although the experimental lamp described contained a 35 00 white phosphor of zinc beryllium silicate and magnesium tungstate, the phosphor for commercial use may be any one which efficiently uses and, therefore, has a high absorption of, ultra-violet radiations in the region of 2537 A. U., that is, the mercury resonance radiation, and consequently a strong ultra-violet response giving a good light output. Another example of phosphors which may be employed are the halo phosphates described in theBritish Patent No. 578,192.

Although preferred embodiments have been disclosed, it will be understood that modifications may be made within the spirit and scope of the invention.

We claim:

l. A fluorescent lamp comprising an elongated phosphor-coated translucent vitreous envelope, an electrode in each end portion of said envelope, and a contained mixture of about 20% xenon and 80% argon admixed with mercury vapor and at a pressure between l and 4 mm. of mercury so that the quantity of light produced,

7 and efficiency of light generation is approximately at amaxirnum. l A Y v 2. A uorescent'lamp comprising an elongated phosphor-coated translucent vitreous envelope, Van electrode in each end portion of said envelope, and a contained mixture of about 20% Xenon and 80% argon admiXed with mercury vapor and at a pressure of about 1 mm. of mercury so that the quantity of light produced and the efficiency of light generation are near a maximum.

VReferences Cited in the le of this patent UNITED STATESPATENTS 1,726,107 Hertz Aug. 27, 1929 8 Fonda et al Dec. 27, Fritze et al. Apr. 2, Servigne Aug. 6, Johnson Mar. 9, Morehouse Sept. 10, Lysohn June 3, Hultgren Aug. 12, Found June l2, 

1. A FLUORESCENT LAMP COMPRISING AN ELONGATED PHOSPHOR-COATED TRANSLUCENT VITREOUS ENVELOPE, AN ELECTRODE IN EACH END PORTION OF SAID ENVELOPE, AND A CONTAINED MIXTURE OF ABOUT 20% XENON AND 80% ARGON ADMIXED WITH MERCURY VAPOR AND AT A PRESSURE BETWEEN 1 AND 4 MM. OF MERCURY SO THAT THE QUANTITY OF LIGHT PRODUCED, AND EFFICIENCY OF LIGHT GENERATION IS APPROXIMATELY AT A MIXIMUM. 